The lateral giant fiber escape reflex of the crayfish shows marked habituation and is unusually amenable to detailed electrophysiological and anatomical study. It thus provides a model system for comprehending the neural basis for one sort of learning phenomenon. The present research will be devoted to analyzing the central nervous "circuit diagram" of the reflex and to determining at precisely what points in this circuit the transmission failures responsible for habituation occur. Having located these points of blockage, it may then be possible to determine their cause.